1. Field of the Invention
The present invention relates to the structure of a drill which is mainly employed for piercing, and more particularly, it relates to improvements in the tip configuration, strength etc. of a drill. The invention further relates to an improvement in the structure of the connection between an insert and a shank of a throw-away tipped drill, and the structure of a lock screw which is employed for fastening the insert to the shank.
2. Description of the Background Art
A drill is one type of cutting tool which is employed for piercing steel products or the like. FIG. 1 shows an exemplary structure of a conventional twist drill 30. This twist drill 30 is formed by a cutting portion 31 which is employed for piercing, and a shank 32, which does not perform cutting but is mainly employed for discharging chips, to be mounted in a chuck of a cutting machine such as a drilling machine.
FIG. 2 shows the forward end of such a twist drill 30. A pair of cutting edges 33 are arranged at positions that are substantially uniformly circumferentially spaced about the rotation axis of the drill 30. These cutting edges 33 linearly extend from ends of a chisel edge 34 toward the outer circumference of the drill 30.
FIG. 3 shows another exemplary structure of a conventional spade drill 40. This spade drill 40 is formed by a shank 41 and a cutting portion 42 which is fixed to the shank 41 by a mounting pin 43. FIG. 4 shows the forward end of the cutting portion 42. The cutting portion 42 of the space drill 40 has the general shape of a flat plate. The forward end of this cutting portion 42 is formed by a pair of symmetrically provided cutting edges 44, which linearly extend from a central portion toward both edges of the cutting portion 42. The drill 40 is further provided on its surfaces, serving as flanks, with slit-type nick grooves 45 extending in a direction substantially perpendicular to the cutting edges 44.
In the conventional twist and spade drills 30 and 40, the cutting edges 33 and 44 that directly contribute to cutting a workpiece are linearly shaped as shown in FIGS. 2 and 4. When such drills are employed for piercing, chips are continuously formed in widths corresponding to the widths of the linear cutting edges. Continuous formation of such wide chips leads to problems such as chip loading in a drilled hole and chip winding on the drill.
Thus, the slit-type nick grooves 45 are formed in the linear cutting edges 44 as shown in FIG. 4, to reduce the chip widths. Namely, the chips are parted at the nick grooves 45, to be reduced in width. However, the chips thus reduced in width are disadvantageously increased in length since the same are liberated from the inner wall of the drilled hole and a groove of the drill. Such long chips may cling to the drill to extremely deteriorate chip controllability.
In using a conventional drill having such linear cutting edges, the drilled hole may become loaded with the chips and chip controllability may be deteriorated by clinging of such chips. Particularly in the case of deep hole drilling, chip loading disadvantageously causes breakage of the drill.
Further, the conventional drill, such as the twist drill shown in FIG. 1, for example, is not provided with any chip breaker. Thus, long chips that are formed during piercing drilling are wound on the drill or cling to the same, to cause problems in automation and availability for piercing drilling operations. While the spade drill shown in FIG. 3 may be provided with a chip breaker having a grinder, the application range of such a breaker is too narrow to attain a sufficient effect in practice.
A drill is an expendable item having a limited life due to wear or breakage during drilling operations. In consideration of economy, therefore, it is preferable to employ a drill such as the spade drill 40, in which only the cutting portion 42 is exchangeable, for example. However, the cutting portion 42 of the conventional spade drill 40 is fixed by the mounting pin 43. Thus, working accuracy may be reduced by a backlash in mounting, and the drill 40 may be broken due to insufficient mounting strength.
Further, the conventional drill also has the following problem: As shown in FIGS. 2 or 4, the drill is generally provided on its tip with a region called a chisel edge 34 or 46. Since such a chisel edge increases cutting resistance and receives a large thrust during cutting work, this portion may be ground out by thinning so that its edge width is reduced.
On the other hand, the inventors have developed a self-grip type drill as described below, and tried to thin the tip of its cutting portion by means of a conventional thinning technique. FIG. 5 is a plan view of the drill developed by the inventors showing a cutting portion 51, which is thinned in a conventional manner, and FIG. 6 is a front elevational view showing the cutting portion 51. A thinned surface 52 defined by the conventional thinning method is shaped as a curved surface along a cylindrical side surface from a chisel edge 53 toward a rear portion of the tip. The width of the chisel edge 53 is defined by a radius R.sub.1 formed on the upper end of the thin surface 52. If this radius R.sub.1 is increased, then the roundness in a central portion of a tip 54 as well as the width of the chisel edge 53 are also increased, which deteriorates the sharpness of the drill. If the radius R.sub.1 is reduced, on the other hand, a radius R.sub.2 of the thinned surface 52 formed in the rear portion of the tip is also reduced although sharpness of the drill is improved, which defines a relatively steep inflection surface in the rake face of the drill in the form of a groove. This causes stress concentration in the vicinity of the thinned surface 52 and reduces the strength of the drill.